In 1939 20th infantry regiment had a slightly different structure then average Polish infantry regiment. The regiment had full bicycle company for reconnaissance duties and weaker anti-tank company, which had only two AT platoons instead of regular three.

Regimental structure in 1939 with names of the officers in charge:
- Regiment Commander Col. Kazimierz Brozek (until 09/02/39)
- Reconnaissance company: Lt. Zbigniew Borowiczka
- Bicycle company: Lt. Michal Rogowski
- Anti-Tank company: Lt. Julian Krzewicki
- Infantry artillery platoon: Lt. Antoni Mazurkiewicz
- Pioneer platoon: Lt. Feliks Lew
- Anti-gas platoon: Lt. Roman Rutke

I/20 Battalion: Maj. Ludwik Balos (regiment C/O from 09/02/39 until 09/20/39)
- 1st company: Lt. Pawel Sawicki (+09/16/39)
- 2nd company: Lt. Wsiewolod Romanowski
- 3rd company: 2nd Lt. Jerzy Zagorski
1st MG company: Lt. Stanislaw Gawronski

II/20 Battalion: Cap. Waclaw Nowacki, from 09/06/39 Cap. Jan Stanaszek and later Cap. Jerzy Ludtke.
- 4st company: Lt. Bronislaw Osmelak (+09/08/39)
- 5th company: 2nd Lt. Jan Sowa
- 6th company: 2nd Lt. Walery Krokay, from 09/06/39 Lt. Wojciech Wielgus (+09/08/39)
- 2nd MG company: Lt. Julian Kajdy, later Cap. Jerzy Ludtke

III/20 Battalion: Maj. Tytus Brzosko, from 09/16/39 Cap. Stanislaw Zieba
- 7th company: Lt. Stanislaw Kosmala
- 8th company: 2nd Lt. Jan Daniec, from 09/06/39 ??? Fara.
- 9th company: Lt. Roman Scholl, later ? Jozef Ignacy Mucha.
- 3rd MG company: Cap. Jan Bujwid, from 09/01/39 2nd Lt. Marcin Koterbicki.

20th Infantry Regiment was part of 6th Infantry Division from Krakow under gen. Bernard Mond. 6th Infantry Division was part of Army "Krakow" and regiments primary goal in the defense plan was to hold area of Pszczyna as long as possible.

Third battalion of the regiment was mobilized in March of 1939 and in July was sent to Pszczyna area to organize defenses, consequently on the September 1st this battalion was the first one to face German armored onslaught. I and II battalions of the regiment were mobilized on August 24th, within couple of the days both battalions reach full strength and by August 31st both reached its destination near Pszczyna and were placed in divisional strategic reserve.

On September 1st 1939 thick mist, in front of the positions of third battalion, significantly slowed down German attack. However, once mist disappeared, around 11:30 AM, Germans moved forward and third battalion came under constant attacks during that day. Germans several times managed to breach Polish positions but each time were repulsed by Polish counter attacks. By midnight III battalion was ordered to withdraw from its positions to the main line of defense. As mentioned above III battalion was part of first line of defense and as such was engaged in heavy defensive battle against German tanks and motorized infantry from 5th Armored Division.

On September 2nd all battalions of the regiment were grouped around Pszczyna and prepared to defend that city with I battalion held in reserve. Around 10:30Am, III battalion was attacked by around 150 tanks from 5th Armored Division. German tanks wiped out both 7th and 8th companies of the battalion within minutes. 9th company found refuge in woods adjacent to Wisla River and held there until nightfall. At the same time II battalion was holding its positions south of Pszczyna not being aware of the catastrophe that took place farther north. Despite repeating attacks II battalion hold its positions until nightfall. At the same time I battalion was being held in reserve unaware of situation on the font line. Lack of coordination among battalions was caused by fact that col. Kazimierz Brozek was cut off from his regiment at the Divisional HQ and remained there without contact with his troops for the rest of the day. As a result of it all battalions fought independently with out any coordination and suffered heavy casualties, except for I battalion, which was not engaged on that day. By the nightfall all remains of the 20th regiment began difficult retreat behind Wisla River.

During the night of September 2/3 regiment was retreating east towards Krakow, its home. While in Krakow, during September 3rd and 4th, regiment reorganized and received reinforcements. During those two days II and III battalions were reorganized and again were combat ready units. Unfortunately, due to general retreat of Army "Krakow" 20th regiment had to abandoned Krakow on September 5th and retreat farter east toward Dunajec River. Polish HQ anticipated that the line of the Dunajec River will be hold and Germans will be stopped there.

Unfortunately, regiment did not have a chance in a race with motorized enemy. By September 7th regiment reach western banks of Dunajec River only to find out that Germans are already holding the bridgehead on the river in Biskupice Radlowskie! At down on September 8th three battalions of the 20th regiment along with two battalions of 3rd Mountain Infantry Regiment from 21st Infantry Division, mounted attack on the Germans holding the bridge. Initial success however, did not last for too long as Germans brought reinforcements that attacked II battalion of the regiment from the back. During that fatal battle both 4th and 6th companies of the II battalion were decimated and both commanding officers were killed (Lt. Wojciech Wielgus and Lt. Bronislaw Osmelak) Germans brought later even more reinforcements and the battle started slowly turning into slaughter. At least 250 soldiers were killed and over 500 wounded on that day. Finally in afternoon Maj. L. Balos, who was regimental commander since 09/02/39, received orders to stop the battle and ford Dunajec River at Otwinow and Nieciecza. What was really ironic that Polish sappers blew up bridge at the Biskupice Radlowickie before Germans reach it. So even if the battle would be successful, Poles would find that they fought for bridge that was already destroyed!

Once again plans of stiff defense on the river line were spoiled by rapid German advance. Soldiers of the regiment instead of day or two for resting had to continue retreat east with Germans right behind them and in some instances even ahead of them. During September 9th and 10th regiment continued retreat trough Szczucin, Baranow, Tarnobrzeg, Nisko to Zdziary. At Zdziary regiment received unexpected reinforcements from fortress battalion that was raised in Krakow (250 men and 5 HMG) as well as 200 stragglers from other units all those men were used to increase strength of II and III battalions. After reorganization at Zdziary regiment still had about 60% of the original men power. For example I battalion had 640 men, 21 LMG, 12HMG, 3 AT guns. II Battalion had around 360 men and 3HMG, III battalion had around 500 men and 11 HMG. During that reorganization, regimental units were also recreated; pioneers platoon, AT platoon and mounted scouts platoon.

September 12th was again marked by very demanding marches. Retreating regiment passed trough completely destroyed Janow Lubelski and during 13th continued retreat farter northeast towards Bilgoraj and Lipowiec. At that time German 14th Army was encircling entire Army "Krakow". So not surprisingly soldiers of the 20th regiment found more and more German troops blocking their way eastward. During September 14th -15th lead elements of the regiment were heavily engaged with German 28th Infantry Division in combat for crossings on the River Tanew. During two days of fights at least 104 soldiers form the regiment were killed and at least twice as much were wounded. During the night (September 15) Poles received information that next day Germans are planning strong attack. Entire regiment took defensive positions along River Tanew, in old remembering WWI trenches. At 6 Am German heavy artillery fire started and caused the forest behind Polish lines to catch on fire. Smoke from burning forest created perfect cover over Polish lines. Therefore, German artillery fire for most part proved very inaccurate but it lasted until noon. Heavy fight with German 28th Infantry Division lasted most of the afternoon and eventually Poles were forced to retreat. During the retreat commander of the 1st company Lt. Pawel Sawicki was killed and commander of the III battalion Maj. T Brzosko was wounded. 20th Regiment was covering the retreat of the entire division with its III battalion being the last one the retreat.

During September 17th regiment spent all day fighting with Germans in vicinity of Jozefow in hopes of cutting trough German lines toward Tomaszow Lubelski were entire Army "Krakow" was also trying to open German encirclement. Despite heavy losses and bravery soldiers of the regiment did not manage to cut trough German defenses. General Mond, commander of 6th Infantry Division, decided to try to open German encirclement at Narol. He order 20th regiment to take lead and attack Werchrate at down on September 18th. The attack was successful and Germans were pushed out of Werchraty but they regrouped behind that village and stopped further Polish attacks. The only road to Rawa Ruska and Lwow were still blocked.

On September 18th troops were reached by News that Soviets crossed Polish borders. This news caused some depression among troops since no one was sure why Soviets corossed Polish borders but to many it was obvious that Soviets are taking adventage of Poland's difficult situation.

By September 20th, 6th Division was completely encircled and did not have any communication with the rest of the army but gen. Mond was still hoping that his division will break trough German lines and will reach Lwow. He planned one more attempt to break trough German lines at Cieszanow. By the time 20th regiment, or what was left of it, reached Cieszanow, German emissary arrived to Gen. Mond. Germans demanded capitulation of his troops or meet certain annihilation. Gen Mond called last meeting and after listing to the reports from his officers he decided that further fight will only cause unnecessary losses and will not make difference for the final out come of the campaign. For campaign of 1939, 22 officers and men were awarded Virtuti Militari Cross and 54 Officers and men were awarded Cross of Valor.